1. Field of the Invention
The invention relates generally to the deposition of transparent conducting thin films on flexible substrates by pulsed laser deposition (PLD), and more particularly to the deposition of thin films such as transparent conducting oxides (TCO) on flexible substrates.
2. Description of the Related Art
Tin doped indium oxide (ITO) and aluminum doped zinc oxide (AZO) thin films, because they combine both transparent and conducting properties, have been widely used as transparent conducting electrodes in optoelectronic devices such as solar cells and flat panel displays (FPDs), surface heaters for automobile windows, camera lenses and mirrors, as well as transparent heat reflecting window material for buildings, lamps, and solar collectors. They are also widely utilized as the anode contact in organic light-emitting diodes (OLEDs). There are several deposition techniques used to grow these TCO films, including chemical vapor deposition (CVD), magnetron sputtering, evaporation, and spray pyrolysis. These techniques require either a high substrate temperature during deposition or a post deposition annealing treatment of the films at high temperatures. These high temperatures generally damage the surfaces of both the substrate and the film. In the typical organic light emitting diode (OLED) geometry, the TCO films are used as the anode contact and are deposited directly onto the transparent glass substrate. In some cases, it is desirable to reverse the device geometry in which case the TCO film would have to be deposited on top of the organic emitting layer. In this case the sputtering technique cannot be used to grow the electrode film because the energetic species ( greater than 100 eV) from the sputter target damage the organic layer. This limitation can be overcome by using PLD to deposit the top electrode because the PLD has low energy species due to high background gas pressure.
Glass substrates have been widely used for the development of OLEDs. However, glass substrates are unsuitable for certain applications such as electronic maps and portable computers. Where flexibility or safety issues are important glass is very brittle and cannot be used since it cannot be easily deformed, or is too heavy, especially for large area displays. These disadvantages can be overcome using either plastic or thin metal foil substrates, which can be very lightweight. To develop an advanced OLED technology based on plastic or metal foil supports requires the TCO material to be either grown directly on plastic or on top of the organic emitting layer for metal foil geometry. Passive and active matrix displays such as liquid crystal displays (LCDs) and organic electroluminescent displays will benefit greatly from the use of flexible substrates.
Recently, the growth of ITO films on plastic substrates by sputtering has been reported by T. Minami et al., Thin Solid Film, Vol. 270, page 37 (1995), and J. Ma, Thin Solid Films, vol. 307, page 200 (1997) (both incorporated herewith by reference) with a rough surface morphology (approximately 6 nm of RMS surface roughness) and a high electrical resistivity of 7xe2x88x9220xc3x9710xe2x88x924 xcexa9-cm. The rough surface morphology and high resistivity of the sputter-deposited ITO films significantly degrade the performance of the OLED.
The current method of depositing TCO films on plastic substrates by sputtering produces a rough surface morphology and high resistivity, which degrades the performance of the OLED. Therefore, there is a strong need for transparent conducting thin films on flexible substrates which exhibit a smooth surface, high optical transparency and low electrical resistivity, which are suitable for use in OLEDs and methods of producing same.
According to the present invention, the foregoing and other objects are attained by providing TCO thin films on flexible substrates which exhibit a smooth surface, higher optical transparency and lower electrical resistivity and methods of making same.
It is an object of the present invention to provide methods for depositing TCO films on flexible substrates to produce a surface with a smooth surface, higher optical transparency and lower electrical resistivity.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing a transparent conducting thin film on a flexible substrate and method for depositing transparent conducting films on flexible substrates
The present invention comprises a TCO thin film deposited on a flexible substrate such that it produces a surface with a smooth surface, higher optical transparency and lower electrical resistivity.
The present invention comprises a method of depositing TCO films on flexible substrates using pulsed laser deposition.